Foo Fighters are clearly enjoying their week-long residency on the 'Late Show With David Letterman.' Not only are they bringing in a variety of guest stars and showing their musical range on a nightly basis, but they've also gotten in on the humor that comes with Letterman's late night show.

Wednesday's appearance found the band showing they could branch off into some good ol' outlaw country and blues with the assistance of Tony Joe White. The band primarily backed up White on the song 'Polk Salad Annie,' though Grohl took the lead on a couple of verses. But White, sitting throughout the performance, got a chance to show off some wicked licks while also displaying his harmonica skills as well.

White's appearance follows the guest turns of Zac Brown and Heart's Ann and Nancy Wilson earlier in the week. During the Monday show, Foo Fighters teamed with the country superstar Brown on a cover of the Black Sabbath metal classic 'War Pigs.' The Wilson sisters played Tuesday night, taking on 'Kick It Out.'

As stated earlier, Foo Fighters also showed off the humor that they've become known for in their music videos as they appeared in a fake commercial for Letterman. In the clip, the band members appear as a law firm ready to fight foo for you. Check out the spoof in the video below.

Foo Fighters' weeklong residency is leading up to the premiere of their 'Sonic Highways' HBO documentary series, which premieres at 11PM ET on the network this Friday (Oct. 17). For the series, the group traveled to some of the best known music scenes in the U.S. and during the trip, they also recorded their new 'Sonic Highways' album. It's due Nov. 11.

Watch Foo Fighters' 'Late Show With David Letterman' Fake Law Firm Commercial